1. Field of the Invention
The present invention relates to magnetic particles of high coercivity, and, more especially, to such magnetic particles comprising ferrimagnetic iron oxides doped with cobalt, to processes for the preparation thereof and to various uses therefor.
2. Description of the Prior Art
Magnetic iron oxide particles doped with cobalt on the surfaces thereof have a higher coercivity than the conventional magnetic iron oxide particles, which do not contain cobalt. Such particles possess improved magnetic properties but typically are characterized by fairly high cobalt levels, and thus are quite expensive. And if the cobalt level of such particles is maintained fairly low, it is necessary to have the iron in the divalent state in order to preserve the magnetic properties; but the particles are then likely to lose these magnetic properties by oxidation of the ferrous iron.
Furthermore, various methods are known to this art for the preparation of magnetic iron oxide particles containing cobalt. Thus, Japanese Patent No. 48-44,040/1973 describes a process including the hydrothermal reaction of a magnetic iron oxide in an alkaline solution, in the presence of a transition metal salt.
Japanese Patent No. 50-85,612/1975 describes the preparation of a magnetic iron oxide by deposition of cobalt onto acicular iron oxide particles and then treatment of such particles with ferrous ions. Also, published French Patent Application No. 2,387,912 describes the preparation of a magnetic iron oxide by partial reduction in gaseous or liquid phase and then by deposition of cobalt onto the acicular iron oxide particles in an alkaline medium.
All of the aforesaid processes include the treatment, in the aqueous phase, of acicular iron oxide particles, (FeO).sub.x.Fe.sub.2 O.sub.3 (with 0.ltoreq.x.ltoreq.1.5), with cobalt complexes which are obtained in an alkaline medium and which attach themselves to the surfaces of the particles. A treatment of this "wet method" type exhibits a very considerable number of disadvantages. In fact, it is difficult to disperse the iron oxides in solution; the deposits of cobalt or ferrous iron on the surfaces of the particles are not homogeneous; and a mixture of the various oxidation states of cobalt (Co.sup.2+,Co.sup.3+) may exist, or Co(OH).sup.+ complexes or inert Co(OH).sub.2 precipitates may be present, on the surfaces of said particles. Furthermore, it is difficult to fix the percentage of ferrous iron when the latter is added.
To overcome the majority of the disadvantages which are attendant a "wet method", German Pat. No. 2,242,500 has proposed to carry out comparable treatment of iron oxide particles by a "dry method". In accordance with this process, carbonyl compounds of cobalt and iron are deposited in the vapor phase, in a fluidized bed, onto acicular iron oxides. However, this process gives rise to pyrophoric products which are inconvenient in the application thereof.